The invention relates to a display tube comprising an electron gun system, a longitudinal axis, a display screen and an electromagnetic deflection unit, which unit comprises a line deflection coil system and a field deflection coil system having two field deflection coils lying opposite each other, with respect to the longitudinal axis, each field deflection coil having a lying gun-sided lobe and with respect thereto upstanding, screen-sided lob with a window in between.
In monochrome display tubes the electron gun system is adapted to generate one electron beam, whereas in, for example, colour display tubes of the in-line type the electron gun system is adapted to generate three coplanar electron beams which converge on the display screen.
The electromagnetic deflection unit for deflecting electron beams is used for deflecting the electron beams in two orthogonal directions from their normal undetected straight path so that the beams impinge upon selected pixels of the display screen so as to provide visual indications on this screen. The electron beams can be moved up or down or from left to right across the (vertically arranged) display screen by suitably varying the magnetic deflection fields. A visual presentation of information or a picture can be formed on the display screen by simultaneously varying the intensity of the beams. The deflection unit, which is secured to the neck portion of the display tube, comprises two systems of deflection coils for deflecting the electron beams in two directions which are transverse to each other; a line deflection coil system to which a line frequency signal is applied during operation and a field deflection coil system to which a field frequency signal is applied during operation. Each system comprises two coils arranged with respect to the tube axis at positions facing each other.
An annular core of magnetizable material surrounds the systems of deflection coils in the conventional manner for concentrating the deflection fields and for increasing the flux density in the deflection area.
To satisfy given requirements of picture quality, magnetic field components of a higher order are often to be added to the (dynamic) magnetic dipole deflection fields. For example, the increasingly stringent requirements of convergence in three-in-line colour television systems necessitate a strong positive magnetic six-pole component at the gun side of the vertical deflection field in addition to a strong negative magnetic six-pole component in the central area of the vertical deflection field. The strong positive six-pole component is necessary for field coma correction. (The effect of a positive six-pole component on the dipole deflection field is a pincushion-shaped field variation.) For a self-converging in-line colour system with green as the central beam and red and blue as the outer beams the field coma is understood to mean a vertical offset of the red and blue beams with respect to the green one. If no coma correction measures are taken, red and blue will be deflected to a stronger extent than green. In the case of a pincushion-shaped deflection field at the gun side red and blue have a weaker deflection field than green. Consequently, red and blue will be deflected to a lesser extent.
Roughly speaking, a pincushion-shaped field is generated in the deflection area when the two coils of a system of deflection coils have broad window apertures, whereas a barrel-shaped field is generated when they have small window apertures. The window of a coil is defined by the internal area which is bounded by the two lateral winding packets at both sides of the tube and the transversal connection packets connecting the lateral winding packets to each other. For a self-converging system particularly the vertical deflection field in the central area must be barrel-shaped (the separate field deflection coils must thus have a small window aperture at that area), and pincushion-shaped at the gun side (i.e. a large window aperture), while it must be homogeneous, more pincushion-shaped or less pincushion-shaped at the screen side, dependent on the quantity of admissible east-west raster distortion. Similar field shapes are also important for monochrome systems of display tubes and deflection units which must have a high resolving power.
Until now it has been found impossible to manufacture deflection coils having a window aperture which varies as much as is desired for said applications, while using the conventional winding methods. However, there are different compromise solutions to alleviate the problem. For example, it is common practice to correct coma by means of plates of a soft-magnetic metallic material arranged at the gun side in the vertical deflection field, which plates enhance the pincushion shape of the vertical deflection field in situ. The use of metal plates in the deflection field is, however, undesired if the display tube deflection unit is to be operated at higher frequencies (EVTV, HDTV). In fact, the energy generated by eddy currents in the metal plates cannot be dissipated in a simple manner so that the temperature of the deflection coil(s) may become inadmissibly high.